1. Field of Invention
The present invention relates to a novel method of and apparatus for transferring heat using heat exchanging fluids that are safely isolated from the environment above and below the Earth's surface and circulated within a sealed heat exchanging structure so as to improve the heat transfer performance of aqueous-based fluid heat transfer systems, wherein the ground, a lake, a river, or sea water is used as the primary or secondary heat sink or heat source in the sealed heat exchanging structure.
2. Brief Description of the State of Knowledge in the Art
The development of refrigeration processes, associated equipment and two-phase chemical refrigerants evolved primarily from the need of mankind to preserve food. Several different kinds of heat transfer systems have been developed for dissipating heat removed from the food to the exterior of the food storage container.
One kind heat transfer system is a typical refrigeration system which includes an evaporator for absorbing heat from one location, a condenser for dissipating heat to another location, a compressor for compressing the vaporous two-phase refrigerant exiting the evaporator for delivery into the condenser where the refrigerant is condensed back into a liquid, and a two-phase throttling device connected to the evaporator inlet for receiving the liquid refrigerant and refrigerant expansion, to complete a refrigeration cycle.
Condensers can be constructed in various configurations, namely: as a tube with air-cooled fins, or as a water-cooled tube and shell configuration. In the water-cooled tube and shell condenser, the rate of heat transfer between the refrigeration-sealed system refrigerant and the water flowing around the tube and shell condenser tube is much higher than the rate of heat transfer between the refrigeration-sealed system refrigerant and air flowing around the tubes of the air-cooled fin and tube condenser.
A water-cooled tube and shell condenser is normally connected with pipes to a cooling tower and a water pump. The heat is absorbed by the water while circulated through the condenser. The heat in the water entering the cooling tower is then dissipated into the atmosphere from the water completing a closed-loop water-cooled refrigeration process.
Environmental concerns have caused strict restrictions to be placed on water-cooled tube and shell condenser systems utilizing a water pump to gather water from other sources such as a lake, a river, sea water, and other fluid systems to be circulated through the water-cooled tube and shell condenser of heat transfer systems. Environmental contaminations vary but are mostly related to chemical concentrations and temperature variations being dispensed into the water source.
A water-cooled tube and shell condenser can be connected with pipes to a ground-source heat transfer well which is used to dissipate heat into the Earth. In various manufacturing processes, the required operating temperature and capacity or volume of heat transfer fluid circulated through the ground source heat transfer well, may not require adding refrigeration to the system.
Residential and commercial comfort air conditioning systems using air-cooled condensers are well known in the art and are used extensively world-wide on air conditioners including heat pumps. Water-cooled tube and shell condensers are normally used in large tonnage commercial and industrial applications such as high-rise buildings, natural gas dehydration, and liquefied natural gas gasification systems.
A heat pump, originally called reverse refrigeration, reverses the refrigeration process through the use of sealed system valves and controls causing the evaporator to dissipate heat while causing the condenser to absorb heat. In its cooling mode of operation, a heat-pump air conditioning system will dissipate heat into the Earth while, and absorb heat from the Earth in its heating mode of operation.
Over the years, ground/water source heat pump has proven very useful as a very efficient form of heating and cooling technology. The use of ground/water source type heat pumps have three distinct advantages over air source type heat pumps, namely: during the peak cooling and heating seasons, the ground/water source usually has a more favorable temperature difference than the atmospheric air; the liquid-refrigerant exchanger on the heat pump permits a closer temperature approach than an air-refrigerant exchanger; and there is no concern with frost/snow/ice/dirt buildup or removal on the heat exchanger.
In general, prior art heat pump installations have employed undersized ground loops because refrigerant-based fluids can provide a sufficient temperature difference between the fluid and the ground so that enough heat is transferred to and from the ground to match the heating/cooling load on the heat pump; however, the use of undersized ground loops is also known to reduce the SEER rating of the heat-pump system. Also, the design goals of prior art heat pump systems have been to minimize the length of the metal pipe used in the ground loop, while just passing the minimum standards for efficiency.
When prior art heat pump systems experience peaks or spikes in heating/cooling load during daily operation, thermal storage solutions are oftentimes added to the system to average the load over the time period of interest. Thermal storage solution also help reduce the cost of the ground loop by allowing the loop to be sized for the average base load over the day, week or season. In fact, many large buildings and residences use thermal storage solutions in order to reduce the cost of heating and cooling by (i) using less expensive night-time electrical loads to heat/cool the thermal mass, and then (ii) using the thermal mass to heat/cool the building during the day. In order to reduce capital cost of the heat pump system, prior art heat pump system installations often use the metal rebar in the foundation or piling as a major part of the thermal mass of the ground loop of the heat pump system.
Ground source or water source type heat pumps can use a closed or open loop as a heat exchanger. Open loops include water circulated to cooling towers; water circulated between wells, geothermal steam wells, water circulated in a body of water such as a river or lake. Closed loops include aqueous based fluids and refrigerant based fluids circulated in cooling/heating coils that transfer heat to air, water, and ground. Most power plants use at least one open loop to generate steam (the burner exhaust) and one open loop (cooling towers or lake) to condense the steam back to water. The de-ionized steam source water is preserved in a closed loop to prevent scale buildup in the heat exchanger. Most conventional refrigerators, freezers and air conditioners use a closed loop of refrigerant to cool the load and an open loop of external air to condense the refrigerant.
The shortcomings and drawbacks of using air to transfer heat from the condenser coil is that air requires a high temperature differential and a large condenser coil surface area to get reasonable heat transfer rates. The high temperature differentials translate to a high-pressure differential which implies higher energy costs to transfer a unit of heat. When a heat pump uses a liquid, from a water or ground loop, to transfer heat from the condenser coil, a smaller coil and a lower temperature and pressure differential can be used to transfer the same unit of heat as the air cool condenser coil which, in turn, improves efficiency and reduces energy costs.
When closed loops are used in the ground or water source of a heat pump system, there is a trade off between using (i) metal tubing with a high heat transfer coefficient (i.e. which is subject to corrosion and thermal expansion), and (ii) plastic tubing with a low heat transfer coefficient (which is resistant to corrosion and thermal expansion). For average soil conditions, plastic tubing usually will require 3 times the heat transfer area of the metal tubing to maintain an equivalent heat transfer rate. Metal tubing is usually reserved for refrigerant based fluids due to the high fill pressures and the reactivity of the refrigerant with plastic tubing.
While protective coatings and grouting can reduce the corrosion rates of metal tubing, pin holes in the coating or grout can actually concentrate the anode corrosion rate in the pin-hole area. Electrical measurements have shown that circulating aqueous based fluids between the ground loop and heat pump can cause the flow of a low level current between the building and the ground.
In accordance with convention, a closed-loop ground/water source heat pump can use a refrigerant based fluid or an aqueous based fluid. With refrigerant based fluids, the heat pump can use a high differential temperature to transfer heat between the ground and the fluid in the tubing, but extra energy load reduces the SEER rating of the heat pump system. Metal tubing is used to contain the pressurized refrigerant based fluid and minimize the volume of refrigerant in the ground loop system due to the high heat transfer coefficient of the metal. As discussed in U.S. Pat. No. 5,025,634 to Dressler, refrigerant based fluids have very high maintenance cost when a small leak develops in the ground/water loop and a very high environmental impact when there is a release of the refrigerant. Also, over a long period of time, field experience has shown that high pressure head loss can develop in the closed ground/water source loop when lubricating oil from the compressor collects low spots in horizontal loop or at the bottom of the bore hole in vertical loop. The inventors design goal was to use an aqueous based fluid in the ground loop to overcome the environmental risk and maintenance problems with refrigerant based fluids.
With most aqueous-based fluid ground/water source loops, the heat pump uses a small closed-loop refrigerant heat exchanger to transfer heat to or from the aqueous fluid. The small heat exchanger reduces the capital cost of the heat pump and reduces the chances of refrigerant releases to the environment. In areas with ground movement, such as earthquakes zones, subsidence bowls, and deep freeze/thaw zones, the borehole thermally-conductive flow guide tube and transfer piping can develop leaks due to repeated damage over time as discussed in U.S. Pat. No. 4,993,483 to Kurolwa. The inventors' design goal was to use a judicious choice of components in the aqueous base fluid; so that, the environmental impact of a large leak can be reduced to non-hazardous spill and the impact of a small leak would be reduced to addition of make up fluid to the loop.
Ground loop installations vary from trenched horizontal loops to multiple bore holes. As disclosed in U.S. Pat. No. 4,644,750 to Lockett and Thurston and in U.S. Pat. No. 4,325,228 to Wolf, a horizontal ground loop's performance is affected by fluctuation in atmospheric surface temperature and soil moisture content, whereas, the ground loop based on multiple bore holes has a stable fluid temperature and heat transfer coefficient for both heating and cooling thermal loads. For heat and cooling loads located on small land surfaces or arid land, the ground loop heat exchanger based on multiple bore holes can provide a heat pump with a stable heat sink or source as described in U.S. Pat. No. 4,392,531 to Ippolito.
The first major improvements to ground loop fluid heat transfer using metal tubing and refrigerant based fluids were disclosed in U.S. Pat. No. 5,816,314 to Wiggs et. al, U.S. Pat. No. 5,623,986 to Wiggs, U.S. Pat. No. 5,461,876 to Dressler, U.S. Pat. No. 4,867,229 to Mogensen, and U.S. Pat. No. 4,741,388 by Kurolwa where metal tubing was bent into a helix shape to increase heat transfer between the refrigerant and the ground. The five patents show that the ‘vertical spiral heat exchanger’ or the ‘bore-hole spiral heat exchanger’ provides the heat pump with a stable heat sink or source for heating and cooling. The shortcoming of these designs is the increased capital cost of spiral bending of the tubing and the increased installation cost of trying to run spiral bent tubing in a deviated well.
Another popular technique used in prior art heat pumps involves insulating the metal, fluid-return tube from the bottom of the bore hole so to prevent heat transfer from incoming fluid, which significantly improves the heat exchanger performance. The deficiency of prior art insulating methods has caused a significant increase in installation costs and a significant increase in capital cost associated with insulating materials. Notably, as the return line was far enough away from the loop to not cause any significant thermal interference, insulating the fluid return tube was not required for earlier horizontal ground loop heat exchangers.
U.S. Pat. No. 4,741,388 to Kurolwa discloses using a spirally-corrugated outer tube to create the spiral flow shape for increased heat transfer of the fluid, which is similar to the spiral channeled tubes used in a steam boiler.
U.S. Pat. No. 5,623,986 to Wiggs discloses that external spirally shaped fins can be used to drill short vertical heat exchangers into sand-loam soils or mud bottoms, but field experience has shown that there is too much fin damage for hard rock/ground surface.
U.S. Pat. No. 5,937,665 to Kiessel et al., discloses other improvements to refrigerant based groundloops, wherein an air heat exchanger is used in the system to reduce the load on the ground loop.
U.S. Pat. No. 6,138,744 by Coffee discloses using a large storage tank of water in a horizontal ground loop that is continuously replenished by an external water source such as waterwell. This technique involves combining an open water loop and a loose ground loop.
U.S. Pat. No. 6,615,601 by Wiggs discloses combining a solar heating loop and a water evaporative cooling loop to the ground loop so as to supplement the heating and cooling load.
U.S. Pat. No. 6,212,896 to Genung discloses a ground loop with large well bores to make room for a vertical thermal siphon to enhance the heat transfer in the large well bore. The shortcoming of this idea is that the heat is transfered to the thermally-conductive flow guide tube wall with a laminar flow of fluid.
U.S. Pat. No. 6,672,371 to Amerman et al. creates a ground loop by drilling multiple well bores from one pad and using plastic U-tubes for the heat exchanger. By using many plastic U-tubes with low heat transfer in series, an equivalent metal heat exchanger performance can be achieved in the ground loop.
U.S. Pat. No. 6,789,608 to Wiggs discloses a technique for extending the performance of the U-tube heat exchanger by installing an insulating plate between the tubes to make two closed separate half wells with minimal thermal interference between each well.
Thus, while various advances have been made in heat pump system design and implementation, there is still a great need in the art for an improved method of and apparatus for transferring heat from above or below the Earth's surface using a sealed fluid circulation system which may or may not incorporate the use of a refrigeration system, while overcoming the shortcomings and drawbacks of prior art methodologies and equipment.